The pancreas is composed of two distinctly different tissues. The bulk of the pancreas consists of exocrine tissue that produces a fluid that facilitates the digestion of proteins, fats, and carbohydrates. Located in discrete clusters throughout the exocrine tissue are aggregates of endocrine cells that produce the several different hormones, notably including insulin and glucagon. These aggregates, termed Islets of Langerhans, are composed of several different cell types. Among these are alpha cells that secrete glucagon; beta cells that secrete insulin; gamma cells that produce pancreatic polypeptide (PP); and delta cells that secrete somatostatin.
Each of these hormones plays an important role in regulating metabolism. For example, PP is known to stimulate gastric and pancreatic exocrine secretions; somatostatin is known to regulate the secretion of other hormones such as growth hormone, insulin, and glucagon; and insulin and glucagon are known to be critical for regulating serum glucose levels. The dramatic physiological effects resulting from abnormal regulation of these hormones underscores their importance in homeostasis. A notable example of this is diabetes mellitus resulting from insufficient insulin production.